Bone mass - what should it be?

I have these fancy scales that give a BMI reading along with fat percentage, muscle mass, water and bone mass. I can find ideal ranges for everything except bone mass. All I can find for bone mass is some calculation where you are measured according to an average and given a plus or minus score - usually as small as 1 or 2 it seems. My bone mass reading is 5.6 - so what does this mean and what is the ideal range?

I know of know scale that could assess 'bone mass' in any way reliably

Body fat is calculated according to how fast a current passes through your body, so I get fat percentage. Muscle = water in terms of measuring it on a set of scales. I suspect they are promising more than they can deliver?

I have no idea how it thinks it is done! There are lots of similar scales on the market which also advertise bone mass readings. There is definitely a way of measuring bone mass/mineral density because that's how your osteoporosis risk is assessed. And that's what this +/- average reading is about. But that's as much as I've found out so far.